This invention relates to methods and apparatus for determining the disposition of earth strata transversed by a borehole. More specifically, this invention relates to improved methods and apparatus using acoustic pulse energy transmission in fluid-filled boreholes for detecting the angle and azimuthal direction of the dip of earth formations.
It is conventional practice to use a dipmeter for determining the angle and azimuthal direction of the dip or inclination of the earth formation strata transversed by a borehole. One common form of dipmeter makes simultaneous resistivity measurements at three or four equally spaced electrodes in a plane perpendicular to the axis of the borehole. The electrodes provide a log of the resistivity of the surrounding formation as the electrode transverse the borehole wall. The logs from all of the resistivity electrodes are correlated in order to derive the relative displacement along the borehole of the points of intersection with the plane of the formation geological dip. Inertial sensors within the dip-meter provide additional information relating to the direction and slant of the borehole and the rotational attitude of the tool itself.
However, such resistivity dipmeters, while effective in boreholes without fluids or in water filled boreholes, have not been particularly effective in boreholes containing oil-base muds. This is primarily due to the insulative quality of the oil-base drilling fluid, since effective resistivity logging depends on the conductivity or resistivity measurement obtained through the drilling fluid and the insulating layer of mud cake adhering to the borehole wall. To overcome this problem to a degree, special "scratcher" and "poller" electrodes have been employed to scrape through the insulating layer of mudcake and then make contact with the formation in order to make the resistivity measurement. Such attempts have the objective to optimize the electrical contact of the electrode with the borehole wall. However, these attempts have met with varying degrees of success, often producing intermittent dip information.
Attempts have been made to use acoustical energy in obtaining information in order to determine formation dip measurements. The need is not for an absolute measurement, but for a log which can determine the change or transition from one earth stratum or formation to another. Attempts have been made to transmit acoustic pulse energy into the formation from a transducer in contact with the borehole wall and then to measure the transmission time of the acoustic energy through the formation to spaced receiving transducers also in contact with the borehole wall. Other attempts have been made to direct the acoustic pulse energy at the borehole wall/formation interface and to measure the acoustic pulse energy reflected therefrom, which would be indicative of the acoustic impedance of the formations. However, in both cases above described, it is not possible to place the acoustic transmitting transducer in continuous direct contact with the borehole wall because of the random wall geometry that occurs during drilling, and any invasion of the oil-base drilling fluid between the transducers and the borehole wall/formation interface will introduce some attenuation in the acoustic pulse energy. It has been found that the heavier the "weight" of the oil-base mud, the greater the attenuation of the acoustic energy, due to scattering produced by the "weighting" material, such as barite or hematite, and the viscosity of the oil-base mud. In addition, in a pulse-echo configuration, the "noise" generated by the ringing decay of the transmitting transducer complicates the detection and accurate determination of the reflected acoustic pulse energy. If a "delay line" spacing is built into the transducer design between the transducer crystal and the borehole fluid, such a "delay line" to delay the transducer ringing also introduces additional reflection and transmission losses. Further, in the first case above described, the "travel" through the formation itself to the receiving transducers introduces additional attenuation. Accordingly, the use of such acoustic pulse energy systems has been limited and the results have not been reliable when attempting to determine the geological dip of the formations traversed by the borehole. Accordingly, the present invention overcomes the deficiencies of the prior art by providing improved methods and apparatus for acoustic examination of the earth formations surrounding a borehole filled with a known fluid and for determining the geological dip of such formations.